MRI sacroiliitis mimics

PURPOSE OF REVIEW: MRI of the sacroiliac joints (SIJ) is frequently performed in diagnostic workup of axial spondyloarthritis (axSpA), especially to detect not only early active inflammatory changes but also structural changes. The presence of subchondral bone marrow edema (BME) is an important early sign of sacroiliitis, but can also occur in many other conditions, whereas structural changes, especially erosions, are more specific axSpA features. The purpose of this review is to describe and illustrate MRI features of the most common conditions in adults that can mimic axSpA sacroiliitis. RECENT FINDINGS: These findings have focused on the frequently occurring anatomical SIJ variations, as their presence can be accompanied by non-inflammatory BME mimicking sacroiliitis. Studies of pregnancy-related changes, osteitis condensans ilii and other strain-related conditions have shown that non-inflammatory strain-related BME is most common in the anterior portion of the SIJ, whereas axSpA changes are more widespread. Awareness of data-driven MRI lesion thresholds with at least 95% specificity for axSpA may facilitate the differentiation between axSpA and the mimics with the exception of osteitis condensans ilii and postpartum changes. SUMMARY: Early accurate detection of axSpA is important for treatment in the clinic and for classification in research settings, and sacroiliitis misdiagnoses can result in inappropriate treatment.